During the drilling of most oil and gas wells located offshore or in inland waters one or more productive zones may be penetrated before the wellbore reaches the primary zone from which the operator wishes to produce oil or gas. These wells are then completed for production from this primary zone, with the intention of later re-completing for production from the other zones after the primary zone is depleted. However, to re-complete a well it has been necessary to pull all the tubing, isolate the original production zone, cement, perforate and reinstall the tubing and production equipment. These operations require mobilization and demobilization of a completion rig, at a cost of one half million to one million dollars or more. As a result, it is often economically infeasible to re-complete to the secondary zones, so wells are often abandoned when the primary production zone is depleted.
In secondary completion methods previously used, cement weighing perhaps 15.6 pounds per gallon is pumped down the annulus between the tubing and the casing to isolate the secondary production zone. This cement plug is then pressure tested, and the production zone is perforated. The perforating gun must fire through the tubing wall, the cement and the casing wall before it can reach the formation. Moreover, only a very small perforating gun can be used in the production tubing. For example the maximum size perforating gun which will fit into a 2-3/8 inch tubing is 1-11/16ths inch in diameter and the maximum size which will fit into a 2-7/8 inch tubing is 2-1/8 inch in diameter. Because of the small size of the perforating gun, there is a limited amount of energy which can be imparted to form the perforations. For this reason, when it is necessary to perforate through the tubing wall, the cement and then the casing wall, there is often very little energy left to provide any significant perforation of the formation. The flow rate of oil and gas through such perforations is much less than through conventional perforations.